Switched-capacitor converters are a class of voltage converters which provide energy transfer and voltage conversion using capacitors. Each leg of a switched-capacitor converter includes a capacitor, and a switch device is connected to each leg for controlling charging of the capacitors. In some implementations, some of the legs also include inductors which makes those legs resonant. In either case, different groups of the converter legs are coupled to different branches of a rectifier at the output. The different groups of converter legs are switched alternately to transfer energy from the input to the output. The rectifier, such as a half-bridge rectifier, rectifies the energy transferred from the capacitors during each switching cycle. The rectified output can be applied directly to a load, or to another converter stage such as a buck converter, a POL (point-of-load) converter, etc.
For a switched-capacitor converter in which each leg is switched with the same duty cycle, the current in each leg ideally is identical. However, due to tolerances of the inductors, capacitors, etc., some or all of the currents in the different legs are phase misaligned, meaning that the current in some legs crosses zero before or after the current in other legs. Even with inductor and/or resonant capacitor tolerances as low as 10%, a significant decrease in converter efficiency occurs due to the phase misalignment of the leg currents.
Ideally, the switch connected to each resonant leg is switched under ZCS (zero-current switching) conditions in which a switch device is turned off when current through that switch device crosses zero and remains off for some dead time. Otherwise, at high current levels under non-ZCS conditions, switching loses are quite due to the high number of switch devices employed. However, standard 50% duty cycle switching does not allow for ZCS in practical implementations where the components have tolerances and other nonlinearities are present, and ZCS is lost for some or all of the legs. Conventional approaches implement complete open loop control, and simply increase the dead time between turning on and turning off the switches of each leg in an attempt to avoid positive or negative switch currents at turn off time. This approach decreases system efficiency, by lengthening each switch cycle with significant additional dead time.